In many rivers a large fraction of traditionally-defined dissolved trace elements is colloidally-bound. To investigate the influence of different molecular weight pools on the bioavailability of trace elements to the zebra mussel (Dreissena polymorpha), animals were exposed to trace elements (Ag, Cd, Se (IV), Hg) in filtered Hudson River water containing: (1) total "dissolved" (<0.2 mm), (2) high molecular weight (HMW; 1 kDa-0.2 mm) and (3) low molecular weight (LMW; <1 kDa) pools. Uptake rates in the total "dissolved" and HMW treatments were similar for Ag, Cd and Hg. Those rates were approximately 7, 10 and 4 times greater than the uptake observed in the LMW treatment. In contrast, the uptake of Se appears to be similar in the different molecular size fractions. This result indicates that Ag, Cd and Hg, but
not Se, were absorbed more efficiently by the animals when colloids were present, contrary to results of previous uptake experiments using phytoplankton. A large fraction of colloidal matter in the freshwater Hudson River is organic. We hypothesize that uptake of colloidal Hg, Ag and Cd is coupled with uptake of colloidal C, which would imply that zebra mussels are able to exploit a fraction of HMW OC. We are currently investigating whether zebra mussels are able to absorb colloidal C and use it as an energy source.